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Why is it hotter inside an isolated car (air conditioning is off) than outside during a sunny day in summer?

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that's greenhouse effect.... :P.... Glass in car window pane function in the same way as $CO_2$ in earth atmosphere.. So, as $CO_2$ traps heat in earth atmosphere and prevents its escape, similarly, glass prevents the escape of heat from car. – Vineet Menon Sep 2 '11 at 7:14

4 Answers 4

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The heating of the car is the same as the heating of the Earth due to the greenhouse effect--- the sun shines through the glass depositing very energetic photons whose equilibrium temperature is 5000 degrees, but the interior surfaces reradiate that light as photons whose temperature is at the interior temperature. The interior end up at a higher temperature, where the outgoing thermal power is equal to the incoming non-thermal solar power.

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The air is transparent to the wavelengths of incident light and is not heated (much) by the sunlight; instead, the interior of the car is heated first. The glass, however, does not as readily transmit the wavelengths of heat radiated from the interior. The absorbed heat increases the car temperature until its heat losses to the environment equal the heat deposited in the car.

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Several effects all push the interior temperature above the ambient air temp. Most discussed, above is that to the extent that the glass is opaque (or partially opaque) to thermal infrared, you get a greenhouse effect. But you also have the fact, that surfaces exposed to direct sunlight (unless it is highly reflective), and much hotter than the ambient temperature of the free air. Surface temperatures are roughly in thermal equilibrium with the average amount of shortwave (solar) radiation, averaged over the course of several days, so the air effectively remembers the previous nights cooling. The air trapped within the car, has poor thermal communication (high thermal impedance) with the air, and so must become considerably hotter than the outside air in order lose enought heat to its surroundings to balance the heat budget. having to transfer heat from the solid interior of the car, to the air within the car, then to the glass, then finally from the glass to the outside air, adds quite a lot of thermal impedance.

But, note, that consistent with the first point, about the radiative equilibrium temperature during the daytime being much hotter than the ambient air temperature, also applies, even if the car had no transparent windows. Consider a matal box shipping container sitting in the sun. The surface will become tens of degrees C hotter than the air temperature, before radiative, and conductive loses become great enough to bring the surface heat budget into balance. The shipping container, will also get very hot, even without windows. And the roof of your car acts in the same manner.

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Here is my guess of some of the relevant factors:

  1. Greenhouse effect: this causes energy to be "trapped" (in a sense) in your car.

  2. Lack of air flow in your car will make heat dissipation much less efficient.

  3. The relatively lower specific heat capacity of the things in your car, e.g. seats, compared to air will heat up faster.

Please comment if there are more relevant points. Also, corrections are welcome! :)

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